780, add a video card

It is probably the weak power supply. The only cards you can add are mid range older hard to find cards that would probably not be much better or even worse than the integrated graphics. Most video cards require at least 300w psu and newer cards require over 400w or much more. Optiplex psu's have 180w - 240w 

 

Optiplex are business models so are not suitable for gaming or upgrading.

Thanks, I tried with an old Geforce 9500, my power supply is 500 W. (BTW, all metric units which are named after a scientist are uppercase. So unless you have a chip on your shoulder for James Watt, please use W.)